Air bag passive restraint systems for protecting automobile and truck occupants in frontal collisions are beginning to be adopted by most of the world's automobile manufacturers. It has been estimated that by the mid-1990's all new cars and trucks manufactured will have air bag passive restraint systems. These air bag systems are designed to protect occupants in frontal impacts.
Many types of crash sensors have been proposed and several different technologies are now in use for determining if a crash is severe enough to require the deployment of a passive restraint system such as an air bag or seatbelt tensioner. Three types of sensors, in particular have been widely used to sense and initiate deployment of an air bag passive restraint system. These sensors include an air damped ball-in-tube sensor such as disclosed in Breed U.S. Pat. Nos. 3,974,350, 4,198,864, 4,284,863, 4,329,549 and 4,573,706, a spring mass sensor such as disclosed in Bell U.S. Pat. Nos. 4,116,132, 4,167,276 and an electronic sensor such as is part of the Mercedes air bag system. In addition, a crush sensing switch has been proposed which discriminates between air bag desired crashes and those where an air bag is not needed based on the crush of the vehicle as disclosed in Breed U.S. Pat. No. 4,900,880. The subject of this invention is a new sensor which has some advantages over the prior art for some applications. This invention is related to copending applications Ser. Nos. 07/480,273 and 07/480.271 filed on even date.
The choice of the sensor technology to be used on a given vehicle depends on where the sensor is mounted. When a car is crashing only certain portions of the vehicle are crushing at the time that the sensors must trigger to initiate timely restraint deployment. A car, therefore, can be divided into two zones: the crush zone, usually about the first 12 inches from the front of the vehicle, which has changed its velocity substantially relative to the remainder of the vehicle and the non-crush zone which is still travelling at close to the pre-crash velocity. To sense a crash properly in the crush zone the sensors must function as a velocity change indicator; that is, the sensor must trigger at approximately a constant velocity change regardless of the shape or duration of the crash pulse. This invention is concerned with frontal crush zone sensors only, and ones that trigger on a constant velocity change for some implementations and where the velocity change function is tailorable for other implementations.
Air damped ball-in-tube crash sensors are inherently velocity change indicators and are the only sensors which have found widespread use for mounting in the crush zone. Spring mass sensors inherently trigger at smaller velocity changes for high deceleration levels and high velocity changes for low deceleration levels and therefore have only found widespread applicability in the non-crush zone locations of the car. Electronic sensors could be designed to function in either manner and thus could be placed either in the crush zone or in the non-crush zone. Although, the preferred implementation of this invention uses air damping, other implementations are undamped spring mass and electronic sensors
Each of these sensors has significant limitations. If spring mass sensors are placed in the crush zone they will either trigger on very short duration low velocity change crush pulses where a restraint system is not needed or they will not trigger on longer duration pulses where a restraint is needed, depending on the particular sensor design and particular mounting location. In addition, since the motion of the mass in the spring mass system is undamped, it has been very difficult to get reliable contact closure on vigorous crash pulses where the mass bounces back and forth many times. To solve this contact problem, spring mass sensors are frequently placed slightly out of the crush zone for frontal barrier impacts. In this case, however, they sometimes become in the crush zone, for example in angle car to car impacts, and are prone to both triggering when a restraint is not desired and to the contact problems discussed triggering when a restraint is not desired and to the contact problems discussed above.
Electronic crash sensors have so far only been used in protected passenger compartment non-crush zone locations. Most electronic sensors have environmental limitations which are exceeded by crush zone locations which are frequently near the engine or radiator. Newer electronic technologies, however, have overcome these environmental limitations and consideration can now be given to crush zone mounted electronic sensors.
The ball-in-tube sensor triggers properly only when responding to longitudinal decelerations. When cross axis accelerations in the vertical and lateral directions are present, the ball can begin whirling or orbiting around inside the cylinder resulting in a significant change in the response of the sensor.
The ball-in-tube sensor depends upon the viscous flow of air between the ball and the tube to determine the characteristics of the sensor. The viscosity of air is a function of temperature and, although materials are selected for the ball and the tube to compensate for the viscosity change, this compensation is not complete and thus the characteristics of the ball-in-tube sensor will inherently vary with temperature. Certain implementations of this invention use viscous air flow and have the same limitations.
In addition, the biasing force which is used to hold the ball at its home position when a vehicle is not in a crash is provided by a ceramic magnet for the ball-in-tube crush zone sensor. This biasing force has a significant effect on the threshold triggering level for long duration pulses such as impacts into snow banks or crash attenuators which frequently surround dangerous objects along the highways. Due to the temperature effects on the magnet, this biasing force changes by about 40% over the desired temperature operating range of the occupant restraint system. Most implementations of the present invention use a spring for the bias thus eliminating this problem.
To function properly, a crush zone sensor of any design must be in the crush zone. Any crush zone sensor which is based on a mass sensing deceleration has a potential of triggering very late if it is not in the crush zone for a particular crash. This is particularly a problem with ball-in-tube sensors which have a very low bias. One example of this involved a stiff vehicle in a low speed barrier impact where the sensor was not sufficiently forward in the car and thus not in the crush zone. The sensor triggered when the entire velocity change of the car reached 10 MPH at which time the occupant was leaning against the air bag. An occupant who is severely out of position and close to the air bag when it deploys can be seriously injured by the deploying air bag. It is therefore important that at least one sensor be in the crush zone for all air bag desired crashes and that all crush zone sensors have sufficient bias to prevent late firing for low velocity long duration pulses. Sensors designed according to the teachings of this invention, generally have a high enough bias that late according to the teachings of this invention, generally have a high enough bias that late triggering is not a problem.
The ball-in-tube sensor is both expensive and subject to wide manufacturing tolerances. This is partially due to the small clearance which exists between the ball and tube. Since this clearance acts as the restrictor to fluid flow, it determines the calibration of the sensor. It therefore must be very carefully controlled. The tolerance on this clearance is typically on the order of 0.000050 inches which requires expensive machining and gaging manufacturing processes. Because of the difficulty in maintaining these tolerances and in particular the tolerance on the roundness of the cylinder, sensors exhibit a manufacturing calibration range of more than 20%!
All crush zone sensors are caused to trigger by being impacted by crushed material moving rearward as the vehicle crushes progressively during a crash. The geometry of this crushed material can vary from vehicle to vehicle and from crash to crash. If a sensor has a shape which causes it to project outward from its support in a cantilever fashion, it is prone to be rotated as it is impacted by the crushed material. In some cases, this rotation can be so severe as to prevent the sensor from triggering since the sensor is no longer pointed forward. A study of crushed vehicles form real life crashes has shown that rotation of the sensor mounting locations is frequently severe. If instead, the sensor has a flat shape with the thickness in the sensing direction small compared with the width and height of the sensor, the local shape of the crushed material impacting the sensor will have a smaller effect, the sensor will have a better support against rotation and the sensor will tend to align itself with the have a better support against rotation and the sensor will tend to align itself with the direction of force thus increasing the probability of properly sensing the crash.
The present invention seeks to eliminate the drawbacks of these other crush zone sensors as explained below.